No, his UE are not all explained by attacking more, far from that.
Actually many of the shots he misses are played with spin, not flat shots.

And he's more often late when the opponent's ball is long than he was in Doha.

Everyone's entitled to his/her own opinion . I think Rafa has surely improved playing more attacking tennis. And you can't compare the Doha surface - medium paced - with the IW/Miami one (very slow).
A winning shot doesn't always need to be a flat one, for instance - Rafa knows very well that a lot of players (perhaps I should say ALL of them) have a lot of problems with his spin balls and as such, they are winners too when the opponent isn't able to return them.